Abstract Detail

The angiosperm genus Myriophyllum L. (Haloragaceae R. Br.) is among the most speciose of the aquatic core eudicots (>60 spp.). The genus has a world-wide distribution with its center of diversity in Australia (39 species; 36 endemic) and North America the second most speciose continent (12 species; 7 endemic). There are three widespread invasive watermilfoil species (M. aquaticum, M. heterophyllum and M. spicatum) that have drawn attention from international natural resource managers. Reliable morphological characters for distinguishing Myriophyllum species have been difficult to define. The problem becomes even more complex when reproductive structures are lacking as is common among these highly clonal plants. Some species that have been assumed distantly related canít be readily differentiated when only submerged vegetative characters are available. Furthermore, the common plasticity (submerged and emergent vegetative forms) found among Myriophyllum only compounds the problem. A molecular phylogenetic approach was taken to examine relationships among taxa and to employ molecular markers for the reliable identification of Myriophyllum taxa. This study included ≈80% of the known Myriophyllum species. Both nrDNA ITS and cpDNA data were used to examine phylogenetic relationships among species. The nrDNA ITS data proved to have high variability and could differentiate all known species of the genus examined while also providing differentiation within some. Phylogenetic analyses resolved two major clades in the genus with several well-resolved subclades. Molecular data also helped uncover previously unrecognized cryptic species among notoriously difficult Australian species complexes.